The subject invention lies in the field of thrombosis and haemostasis. In particular the invention is directed at improved chromogenic substrates for thrombin. The substrates according to the invention can be used to great advantage in determination of endogenous thrombin potential. The determination of endogenous thrombin potential and advantages and applications thereof are described in EP appl.nr. 902025097, published on Apr. 3, 1991, publication number 420332 of Hemker et al. The content of said patent is hereby incorporated by reference. The subject invention is also directed at improved methods of ETP determination in a continuous assay.